Digital high definition video data may be successfully transmitted over terrestrial television channels, as described herein, by partitioning the video data between high and low priority information, and quadrature amplitude modulating the high and low priority data on separate carriers respectively. The modulated carriers are included in a 6 MHZ frequency spectrum and then the combined signal is translated to occupy a standard broadcast channel spectrum. The high priority data is transmitted with relatively high power and the low priority data with relatively low power. High priority data is that video data which is sufficient to reproduce an image, albeit of lesser quality than a high definition image.
Nominally digital video data is compressed and transmitted in layered coded format including header data identifying respective portions of the data so that, upon loss of some data during transmission, the receiver can find an appropriate re-entry point in the received data. In general, however, simply providing header data is not sufficient to enable the receiver to produce acceptable images, since it may be the header data which is lost or corrupted during transmission. In such instances the receiver will tend to lose synchronization with respect to the normal cyclical decoding sequence.
In accordance with the present invention extra protection against lost or corrupted data from confusing decoding at the receiver, is provided by arranging coded video data in transport blocks. The transport blocks include additional header data identifying relatively small portions of the video data. In addition the header data of the transport blocks includes pointers which indicate data re-entry points within respective transport blocks.
The present invention is directed to circuitry for forming video data transport blocks at a video signal encoder and circuitry for depacketizing the transport blocks at a video signal receiver.
For purposes of this disclosure the video data will be presumed to be compressed in MPEG like format. What is meant by "MPEG like" is a coding format similar to the standardized coding format being established by the International Organization for Standardization. The standard is described in the document "International Organization for Standardization", ISO-IEC JT(1/SC2/WG1), Coding of Moving Pictures and Associated Audio, MPEG 90/176 Rev. 2, Dec. 18, 1990, which document is incorporated herein by reference for description of the general code format.
The MPEG standard transmits 240 lines (NTSC) per frame non-interlaced, which is typically accomplished by encoding only the odd or even fields of an interlaced source video signal. For transmitting HDTV signals the standard is modified to provide, for example 480 lines per field, and both the odd and even fields are transmitted. In addition the number of pixels per line is increased to for example 1440. Conceptually these changes only affect the data rate and do not affect the compression philosophy.